Modern wind turbines become larger and larger, and the size of the individual parts of the wind turbines, such as the hub, also increases. In a 3 MW turbine, the blades can be more than 50 meters long and the diameter of the blade root flange can be more than 3 meters. Due to the increasing forces, larger and stronger hubs are needed.
The large parts are difficult to handle and transport from the manufacturing facility to the operating site of the wind turbine.
Typically, the hub is cast in one piece, and the casting process becomes more expensive and the number of supplies reduces as the parts increase in size.
U.S. Pat. No. 6,942,461 discloses a rotor blade hub for a wind power installation. The rotor blade hub is divided into a hub core and a number of outer hub portions corresponding to the number of rotor blades. The outer hub portions are each connected to the hub core and to a rotor blade. The hub core is a relatively large part, and the disadvantages regarding handling and manufacture of large parts are therefore not fully avoided by this hub.
WO 01/42647 discloses a wind turbine rotor hub comprising two shells which are mutually adhered via a plane extending transversely of the rotation axis of the hub. The shells are made from a composite material.
For smaller wind turbines, welded hubs are common. However, in order to obtain stronger blade-hubs for larger wind turbines, they are generally designed as a hollow body defined between two spheres, an inner and an outer sphere, often with their centres slightly displaced to get the optimal distribution of the stress introduced in the hub by the turbine blades. The blade-hub is designed as a hollow body construction to reduce the weight of it and to optimise the strength thereof.
Turbine blades are attached to the hub via flanges and bearings between the flanges allow for pitching of the blades. To reduce weight and to provide access from the hub into the inner cavity within the blades, large openings are typically provided within the blade flange and within the hub flanges.
Due to the openings in the hub, large stress concentrations typically occur in the material in this area during operation of the wind turbine, and deformations may arise as consequence thereof. In response to the stress concentrations, the hub is made with larger and larger material thickness which again complicates manufacturing, handling and transportation of the hub.
Thus, it is an object of the invention is to provide a hub which reduces the complications both regarding stress during operation and regarding manufacturing, handling and transportation of the hub.